As a method of evaluating nuclear properties in the core of a nuclear reactor, there has been a method of comparing design values and actual values (measurement values) about items, such as the output peaking and the axial output deviation of the core. A neutron flux detector is provided in the core of the nuclear reactor and a distribution of reaction rates along the axial direction of the core is calculated on the basis of values measured by the neutron flux detector. By using a plurality of types of data, such as the distribution of reaction rates and the temperature along the axial direction of the core, reactor output distribution processing is regularly performed in order to monitor the core, and the output peaking and the axial output deviation of the core, etc. are calculated. Such methods are described in, for example, the following patent documents.